Implement STRRIN, like STRIN but searching from the right

test/asm/strin-strrin.asm

@@ -0,0 +1,15 @@
+SECTION "Test", ROM0
+
+	assert STRIN("foo bar baz", "bar") == STRRIN("foo bar baz", "bar")
+
+	assert STRIN("foo bar bargain", "bar") == 5
+	assert STRRIN("foo bar bargain", "bar") == 9
+
+	assert STRIN("foo bar", "qux") == 0
+	assert STRRIN("foo bar", "qux") == 0
+
+	assert STRIN("foo", "foobar") == 0
+	assert STRRIN("foo", "foobar") == 0
+
+	assert STRIN("foobar", "") == 1
+	assert STRRIN("foobar", "") == STRLEN("foobar") + 1